Influence of Singlet and Charge-Transfer Excitons on the Open-Circuit

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The Influence of Singlet and Charge-Transfer Excitons on the OpenCircuit Voltage of Rubrene/Fullerene Organic Photovoltaic Device Wei-Cheng Su, Chih-Chien Lee, Ya-Ze Li, and Shun-Wei Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08363 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 7, 2016

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ACS Applied Materials & Interfaces

The Influence of Singlet and Charge-Transfer Excitons on the Open-Circuit Voltage of Rubrene/Fullerene Organic Photovoltaic Device Wei-Cheng Su, Chih-Chien Lee,,* Ya-Ze Li, Shun-Wei Liu,*

Department of Electronic Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Daan Dist., Taipei 10607, Taiwan

Department of Electronic Engineering, Ming Chi University of Technology, No. 84, Gungjuan Rd., Taishan Dist., New Taipei City 24301, Taiwan

*

Address correspondence to [email protected], [email protected]

∥

These two authors contributed equally to this work.

KEYWORDS: small-molecule organic photovoltaic device, open-circuit voltage, charge-transfer states, reorganization energy, singlet exciton

ABSTRACT: We demonstrated that the open-circuit voltage (VOC) of rubrene/C60 organic photovoltaic (OPV) devices can be substantially improved by changing the rubrene thickness. A shoulder exhibited in a range of 500-550 nm was observed. This result indicated that the singlet excitons of rubrene were increased when the thickness of the rubrene layer was increased.

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Capacitance-voltage measurements were conducted for estimating the built-in potential of the devices. The calculated VOC was higher than that of the experiment, thus indicating that energetic losses occurred in the devices. We reused the reciprocity and revised Marcus theory for determining the charge-transfer (CT) properties of the devices. The CT properties of the CT states at the rubrene/C60 interface remained similar. The nonradiative energetic losses become smaller when the rubrene layer was increased, thus indicating the bimolecular recombination was increased. The increased recombination thermally activated the electrons in C60 into rubrene for forming the singlet excitons in rubrene. The reduction in reorganization energy indicated that the electroluminescence of rubrene was enhanced, thereby improving VOC. These results proved that the two-step thermal activation of C60 electrons and the improved VOC of rubrene were caused by the increased singlet excitons of rubrene.

INTRODUCTION The charge-transfer (CT) states of organic photovoltaic (OPV) devices have been extensively studied in recent years because of their determinants on the open-circuit voltage (VOC).1-10 The CT energy (ECT) was found to be highly correlated with the VOC variation between OPV devices that use different materials.2, 4, 6-8 Vandewal et al. reported a series of polymeric OPV devices and the relation between the CT states and VOC. They suggested that energetic losses are unavoidable because of the presence of the CT states at the donor/acceptor interface. The CT states may cause radiative and nonradiative energetic losses that are determined by the recombination dynamics. Before the development of the CT states, it was proposed that VOC is a result of the energy difference between the highest occupied molecular orbital (HOMO) and


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lowest unoccupied molecular orbital (LUMO) levels.11-13 For example, Chu et al. demonstrated an improved VOC through the replacement of C60 with PC60BM that had an up-lying LUMO level for increasing the HOMO-LUMO offset.14 Kinoshita et al. showed that the VOC of pentacene/C60 devices can be improved by inserting a thin layer of copper phthalocyanine having a HOMO level lower than that of pentacene.15 For effectively increasing the HOMO-LUMO offset, Taima et al. introduced rubrene with a low-lying HOMO level as the donor material and improved VOC to over 0.9 V, which is an outstanding value even today.16 Combined with the multilayer structure and the introduction of rubrene, several groups have reported OPV devices with various structures that contain rubrene for achieving a high VOC.17-22 Most results have been explained in terms of the large HOMO-LUMO gap induced by the low-lying HOMO level of rubrene.17-22 In 2007, Pandey and Nunzi discovered that a rubrene/C60 OPV device enabled OPV and organic light-emitting diode properties to be realised.23 They found that electroluminescence (EL) could be observed even when the driving voltage is under approximately 1 V. They proposed a two-step thermal activation for generating rubrene excitons to emit light output. They stated that the CT states recombined at the rubrene/C60 interface can transfer the energy to the electrons on C60. The electrons can cross the barrier at the rubrene/C60 interface, form a rubrene singlet exciton on rubrene, and emit the color of rubrene. Here, we found that two-step thermal activation can indeed form the singlet excitons on rubrene. By changing the thickness of rubrene, rubrene/C60 devices exhibited a large variation in VOC. Analysis of CT states properties showed that the rubrene singlet exciton determines the variation in VOC. This is the first attempt to fit rubrene singlet states for extracting the singlet CT properties. The results showed that the increased rubrene singlet CT states can lead to an increased VOC. This is a new perspective for using rubrene as a donor for increasing VOC.17-22


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RESULTS AND DISCUSSION The

studied

OPV

devices

were

formed

with

a

structure

of

indium-tin-oxide

(ITO)/MoO3/rubrene (5, 10, 15, 20 nm)/C60 (30 nm)/Ag (120 nm). The photo current densityvoltage (J-V) characteristics were measured under AM 1.5 G simulated solar simulator at 100 mW cm-2, as shown in Figure 1a. Table 1 summarizes the photovoltaic parameters. When the thickness of rubrene was increased, VOC was increased from 0.73 and 0.82 and 0.87 to 0.91 V. The fill factor (FF) and short-circuit current density (JSC) differ slightly. Therefore, this experiment allows for focusing on the variation in VOC. Previous studies have suggested that the change in the active layer thickness can affect VOC.24 Built-in potential (Vbi) was proposed as the primary cause of this phenomenon. However, Vbi can also cause differences in JSC and FF simultaneously. Therefore, the Vbi variation does not necessarily dominate the difference in VOC in our study. Other factors influencing VOC are the crystallinity and HOMO level of the rubrene thin film. We investigated these topics with atomic force microscopy (AFM) and low-energy photoelectron spectroscopy, as shown in Section 1 of the Supporting Information. The 5- and 20nm rubrene thin films presented identical surfaces and HOMO levels. Therefore, we can rule out the effects of the crystallinity and HOMO levels do not determine VOC. In addition, our results may indicate that a thicker rubrene layer can improve VOC and PCE values. However, the devices with 30- and 40-nm rubrene layers exhibited poor FF values and fixed VOC values (Supporting Information Figure S2). Figure 1b shows the external quantum efficiency (EQE) spectra of the corresponding devices. Because JSC values were almost identical, the EQE spectra had nearly the same shape and values. One key observation regarding the EQE spectra was that the shoulders were observed in the wavelength range from 500 to 550 nm. This result was observed in a


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previous study,22 but was not discussed in depth. The shoulders were more obvious for specimens with increased thicknesses of rubrene. Because of the limited absorption of the rubrene layer under the photovoltaic process, the increase of the EQE was limited. Although rubrene absorbs a wavelength range of 500 to 550 nm, the absorption coefficient is too low to contribute to the EQE (Supporting Information Figure S3). Therefore, we inferred that the shoulder may result from the rubrene singlet states, as will be discussed later.

(a) 0.5 -2

-0.5 -1.0 -1.5

(b) 40 rubrene 5 nm 10 nm 15 nm 20 nm

rubrene 5 nm 10 nm 15 nm 20 nm

30 EQE (%)

0.0 Current density (mA cm )

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Figure 1. (a) Photo J-V characteristics and (b) EQE spectra of ITO/rubrene/C60/bathocuproine (BCP)/Ag with various thicknesses of rubrene. The error bars are provided on an average of ten devices.

Table 1. Photovoltaic parameters for OPV devices with various rubrene thicknesses. The values are obtained on the average of ten devices. PCE is defined as the power conversion efficiency. Device VOC (V) JSC (mA cm-2) FF (%) PCE (%) RS (Ω cm2) RSH (kΩ cm2) 5 nm 0.73 2.99 60.0 1.31 23.6 1.36 10 nm 0.82 3.05 59.5 1.49 23.5 1.35 15 nm 0.87 3.06 58.7 1.56 24.6 1.42 20 nm 0.91 3.07 58.9 1.65 25.7 1.44


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Figure 2a shows the dark J-V characteristics with various rubrene thicknesses. As proposed in previous studies, the reduced reverse saturation current can lead to a higher VOC.25 Here, only 5nm rubrene may cause the leakage of electrons from the C60 layer. This situation could be eliminated when the rubrene layer was increased, hence, improving VOC, as observed in the previous study.26 In addition, thicker rubrene may increase the possibility of forming rubrene singlet excitons, as observed in Figure 1b. For discussing the influence of Vbi in VOC, we conducted a capacitance-voltage (C-V) measurement to evaluate Vbi in each OPV device. The CV characteristics can be described by the Mott-Schottky relation as follows:27

C −2 =

2(Vbi − V ) A2eεε 0 N A

(1)

where A is the device active area, e is the elementary charge, and NA is the p-type doping concentration. The term Vbi can be estimated from the on-set point. The term Vbi was estimated as 0.57, 0.63, 0.68, and 0.72 V for rubrene thicknesses of 5, 10, 15, and 20 nm, respectively. The difference in device-to-device was approximately identical to the variation in VOC. Therefore, the reduced reverse dark current and the estimated Vbi influence VOC. To investigate the improvement of VOC further, we conducted a temperature-dependent measurement for extracting the barrier height at the rubrene/C60 interface (Supporting Information S4). The barrier height

ΦB was 0.55, 0.58, 0.66, and 0.74 eV for rubrene thicknesses of 5, 10, 15, and 20 nm, respectively. The barrier height was almost the same as the evaluated Vbi. The purpose of deriving ΦB was to calculate the theoretical VOC. The calculated VOC can be obtained using the following equation:28

J  eVOC = nΦB − nkT ln S0   J SC 


(2)

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The term n for all the OPV devices was 1.8 (Supporting Information Figure S5). The calculated VOC was 0.79, 0.85, 0.91, and 0.96 V for OPV devices with rubrene thicknesses of 5, 10, 15, and 20 nm, respectively. The calculated VOC was much higher than the experimental value. This result indicated that energetic losses occur in each device.

(a)

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0.5

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1

0.72 V

0.63 V 0.57 V

0 0.4

0.5

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0.6

0.7

0.8

Bias voltage (V)

Figure 2. (a) Dark J-V and (b) C-V characteristics of ITO/rubrene/C60/BCP/Ag with various rubrene thicknesses.

To understand the energetic losses, the EL and EQE spectra were measured for all OPV devices. They were first determined by a spectrometer (Supporting Information Figure S6). The driving current was fixed at 20 mA. When a thinner rubrene layer was used, the rubrene light output was almost negligible. The 10-nm device exhibited a much higher EL intensity compared to that of the 5-nm device. After increasing the rubrene layer to 15 nm, the EL intensity was beyond the detection limits of the spectrometer. The same phenomenon was observed in the 20nm device. However, the 15-nm device exhibited a shoulder at around 600 nm, thus indicating that the EL intensity of the 20-nm device was much higher than the 15-nm one. These results


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indicated the suppression of the electron leakage current and increased formation of rubrene singlet excitons. For further discussion of the origin of the energetic losses, we reused the reciprocity relation derived by Rau29 and Vandewal et al.30 for obtaining the CT properties such as ECT and reorganization energy (λ). The reduced spectra and energetic losses were obtained as the previous studies (Supporting Information S7).31-33 The reduced spectra for obtaining the CT properties at the rubrene/C60 interface are shown in Figure 3. Table 2 presents the corresponding CT properties and energetic losses. The CT properties, ECT and λ, were identical for all devices. The ECT value of approximately 1.5 eV was different than the value of approximately 1.0 eV reported by a previous study.23 The difference may have resulted from the measurement system used. For a clear definition of the physical meaning of the reported CT energy, we estimated the energy levels of rubrene and C60 as shown in Figure S8. The CT energy was close to the HOMOLUMO gap at the rubrene/C60 interface. Therefore, we attributed the CT energy to the excitons formed at the rubrene/C60 interface. The radiative losses (∆Vrad) were also identical. These losses are unavoidable in a given material system. The nonradiative losses (∆Vnonrad) decreased when the thickness of rubrene increased. We inferred that the electron leakage current causes high bimolecular recombination losses and, hence, the low EL intensity. When the rubrene thickness increased, the electron leakage current was suppressed, increasing bimolecular recombination at the rubrene/C60 interface. However, the CT properties remained the same regardless of the thickness of rubrene. The CT excitons may not play an important role in determining VOC. Previous studies have shown that the bimolecular recombination at the rubrene/C60 interface can thermally activate the electrons across the barrier height at the rubrene/C60 interface.23 Therefore, the formation of the rubrene singlet excitons, the high EL intensity for thicker rubrene layers, and the shoulders caused by the rubrene excitons may indicate that the rubrene singlet excitons


8

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caused the improvement in VOC. Some studies have indicated that the singlet excitons of rubrene can split into triplet excitons.34-35 However, in the current study, we only observed singlet properties in both the photovoltaic and EL processes, as shown in Figure 1b and S7, respectively. Therefore, the discussion of triplet excitons is beyond the scope of the current study. Figure 4 shows the reduced spectra for the rubrene singlet excitons. This is a new finding: the rubrene singlet excitons provide a perfect reduced spectra fitting. As shown in Table 3, the CT energy of approximately 2.3 eV corresponds to the bandgap of rubrene (Supporting Information S8), thus indicating that the reported CT energy was primarily caused by the singlet excitons of the rubrene. Compared with the CT properties at the rubrene/C60 interface, the reorganization energy was reduced when the thickness of rubrene increased. This result may indicate that greater numbers of electrons hopping from C60 to rubrene ease the formation of rubrene singlet excitons. In contrast to the photogenerated excitons under the photovoltaic process, the singlet excitons generated under the EL process can substantially increase once the thermal activation of the electrons increases. An unavoidable ∆Vrad was observed at the rubrene/C60 interface. ∆Vnonrad substantially decreased when the rubrene thickness increased. This result corresponds to the increased EL intensity observed in Figure S7. In addition, the energetic losses were considerably reduced for thicker rubrene layers, thus leading to the improvement in VOC when thicker rubrene was used. These results indicate that the primary cause of increasing VOC by inserting rubrene resulted from the increased number of the rubrene singlet excitons. In addition, fitting the rubrene singlet excitons to yield reduced spectra is a new perspective for investigating the improvement in VOC.


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(b) 10-2 rubrene 5 nm EL EQE

Reduced spectra (a.u)


(a) 10-2

1.513 eV -3

10

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rubrene 10 nm EL EQE 1.510 eV -3

10

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(d) 10-2 rubrene 15 nm EL EQE

10

1.5 Photon energy (eV)

(c) 10-2 Reduced spectra (a.u)

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rubrene 20 nm EL EQE

1.515 eV

-3

10

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1.5

1.6

1.7

10

1.3

1.4

Photon energy (eV)

1.5

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Photon energy (eV)

Figure 3. Reduced spectra of OPV devices with rubrene thickness of (a) 5, (b) 10, (c) 15, (d) and 20 nm at the rubrene/C60 interface. The single-headed arrows along with texts indicate ECT. The double-headed arrows along with texts indicate λ.


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10

0

(b) 101 rubrene 5 nm 2.340 eV EL EQE



(a) 101

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rubrene 10 nm 2.322 eV EL EQE

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rubrene 20 nm 2.300 eV EL EQE

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0.379 eV

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(c) 101 Reduced spectra (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Photon energy (eV)

Figure 4. Reduced spectra of OPV devices with rubrene thickness of (a) 5, (b) 10, (c) 15, (d) and 20 nm for rubrene singlet excitons. The single-headed arrows along with texts indicate ECT. The double-headed arrows along with texts indicate λ.

Table 2. CT properties and energetic losses of OPV devices with various rubrene thicknesses at rubrene/C60 interface. Device VOC (V) ECT (eV) λ (eV) ∆V (V) ∆Vrad (V) ∆Vnonrad (V) 5 nm 0.73 1.513 0.070 0.78 0.191 0.592 10 nm 0.82 1.510 0.070 0.70 0.192 0.503 15 nm 0.87 1.510 0.070 0.64 0.191 0.447 20 nm 0.91 1.515 0.070 0.61 0.192 0.413

Table 3. CT properties and energetic losses of OPV devices with various rubrene thicknesses for rubrene singlet excitons. Device VOC (V) ECT (eV) λ (eV) ∆V (V) ∆Vrad (V) ∆Vnonrad (V) 5 nm 0.73 2.340 0.559 1.61 0.381 1.229 10 nm 0.82 2.322 0.495 1.52 0.382 1.138


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15 nm 20 nm

0.87 0.91

2.319 2.300

0.435 0.379

1.47 1.43

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0.382 0.385

1.088 1.045

CONCLUSION The influences of the CT states at the rubrene/C60 interface and singlet excitons in the rubrene layer were studied in the current study. A large variation in VOC at various rubrene thicknesses resulted from the singlet excitons in the rubrene layer. The bimolecular recombination of the CT states at the rubrene/C60 interface enabled thermal activation of the electrons in C60 and their movement into rubrene for devices with increased rubrene thickness. The heightened thermal activation increased the formation of the rubrene singlet excitons. The increased numbers of the rubrene singlet excitons was found to be highly correlated with the variation in VOC for devices with relatively thick rubrene layer. The current study explained and supported previous studies, which had argued that the use of rubrene can increase VOC in OPV devices.

EXPERIMENTAL SECTION Materials: ITO substrates were purchased from Luminescence Technology Corp. The sheet resistance of ITO was approximately 15 Ω sq-1. MoO3, rubrene, C60, BCP, Ag were purchased from Sigma-Aldrich. All materials were used as received.

Device Fabrication: ITO substrates were cleaned by a standard cleaning process. The thin-film deposition was performed in a high-vacuum chamber with a base pressure of less than 8 x 10-6 Torr. The deposition was appropriately controlled at 0.1 nm s-1 for all thin films and monitored using a deposition controller (Sycon Instruments STM-2XM). The active layer with an active area of 0.04 cm-2 was defined by the deposition of Ag through a shadow mask. A home-designed


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layout enabled the production of five devices on one ITO substrate for avoiding device-to-device deviation. A home-made shutter system allows for fabricating four cell parameters at most without breaking the vacuum for avoiding substrate-to-substrate deviation. After the completion of thin-film deposition, all devices were transferred into a nitrogen-filled glove box. All devices were appropriately encapsulated using epoxy resins (Everwide EXC345) and getter-attached glass for covering the active area.

Characteristics: All measurements were carried out in the air. The film thickness of each layer was calibrated using a surface profiler (Veeco Dektek 3) and ellipsometry (Raditech SE-950). The absorption of materials was measured using a spectrophotometer (Thermo Scientific Evolution 220). Photo and dark J-V characteristics were performed under AM 1.5G solar simulator (Newport 91160A) at 100 mW cm-2 and in the dark. AFM images were obtained using the Park Systems XE-70 in the non-contact mode. An NCHR-type non-contact cantilever with a frequency of 320 kHz, thickness of 4 µm, length of 125 µm, width of 30 µm, and typical tip radius less than 10 nm was used. EQE spectra were measured using a monochromator (Newport 74100) and a lock-in amplifier (Stanford Research SR830) chopped at 250 Hz. C-V characteristics were measured using a precision LCR meter (Agilent 4980A) with a 100 mV ac signal and a frequency of 1 kHz. EL spectra were measured using a spectrometer (Ocean Optics USB2000+). HOMO levels of rubrene and C60 were estimated using a low-energy photoelectron spectrophotometer (Riken Keiki AC-2).


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ASSOCIATED CONTENT

Supporting Information. Crystallinity and HOMO levels of different thickness of rubrene; OPV devices with thicker rubrene layer; the absorption coefficients of rubrene and C60; the evaluation of the barrier height; the ideality factor of OPV devices with various thicknesses of rubrene; EL spectra of OPV devices; methods for fitting the reduced spectra and evaluating the energetic losses; estimation of the HOMO and LUMO levels of rubrene and C60. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge the financial support from the Ministry of Science and Technology (Grant No. MOST 104-2119-M-131-001, 104-2221-E-011-126, 104-2623-E-011-001-D, 1052221-E-131-016, and 105-2221-E-011-065). C.-C. Lee and S.-W. Liu contributed equally to this work. In addition, one of the authors (S.-W. Liu) is grateful to Mr. H.-H. Wu, Syskey Technology Corporation (Taiwan), for his assistance in designing the fabrication system.


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Influence of Singlet and Charge-Transfer Excitons on the Open-Circuit

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